The allegations emerged in a Russian TV report on Sunday and were later backed up by the internal state security service, the FSB.

Ms Alexeyeva, head of the Moscow Helsinki Group, said they were part of a campaign against Kremlin critics, linked to a law tightening control over NGOs which President Vladimir Putin signed this month.

"They are preparing public opinion for a government move to close us down, which they can now do under the new law," she told reporters.

She said a document written in Russian which showed one of the alleged spies' signatures was a fake. All communication between her organisation and British donors was in English, according to Ms Alexeyeva.

'Red-handed'

The FSB backed the claims made on Russia's Rossiya TV channel which showed what it said were British agents retrieving data from the dummy, loaf-sized rock planted on a street.

The device is said to be a hi-tech version of the classic "dead drop"

According to the programme, a UK diplomat made regular payments to Russian non-governmental organisations.

The programme named four individuals it described as British spies working as diplomats and also mentioned a Russian citizen, said to be now in custody after confessing to espionage.

The four were named in the programme as Christopher Pirt, Marc Doe, Paul Crompton and Andrew Fleming.

"The most important thing is that we caught them red-handed while they were in contact with their agents [and established] that they were financing some non-governmental organisations," said FSB chief spokesman Sergei Ignatchenko.

He did not detail what action would be taken by the Russian authorities, saying only that the question would be "decided at the political level". The new law on NGOs bans foreign funding of any NGO with "political purposes" but it does not spell out what this means.